1. Field of the Invention.
The present invention relates to the transfer, storage and delivery of process chemicals. More particularly, the present invention provides improved apparatus and method for the transfer, storage and delivery of ultra-high purity chemicals for use in a variety of industries, such as in the manufacture of semiconductor wafers and similar products.
2. Description of the Prior Art.
In many applications in industry today it is extremely important to maintain process chemicals free of virtually all contaminants. For instance, in the semiconductor industry the purity of chemicals, such as sulfuric acid, hydrogen peroxide, and ammonium hydroxide, used in semi-conductor wafer production must be pure on level of approximately 25 (or fewer) particles per milliliter with a particle size of less than a fraction of a micron. As a result of these purity standards, many conventional methods of chemical transfer and delivery, such as paddled pumps and similar devices, have proven completely unsatisfactory.
Of further concern in these industries is that many of the chemicals employed are toxic and must be carefully handled. In order to assure adequate purity and worker safety, it is extremely important that such chemicals be transferred, stored, and dispensed in a closed system, with minimal contact with the environment or workers.
Generally today one of two methods are employed to effectuate high-purity chemical transfer. The first method is a "pumped delivery." In this method a positive displacement pump, usually an air powered double diaphragm type, is employed to provide both lift at a suction inlet from the bulk source of the chemicals and simultaneous pressure at the output to the enduser. In this system, chemical is lifted from a chemical drum, driven through a pump, and pushed out to the point of use. Although this method is widely employed, it is far from satisfactory.
The deficiencies of the pumped delivery system are manifold. This system is capable of producing only minimal lift from the chemical bulk source--usually on the order of only a few pounds per square inch. Moreover, this system is replete with contamination problems: the rapidly expanding and contracting of the pump diaphragm material (e.g. Teflon.RTM.) causes mechanical degradation, with the degradation by-products (many of which being too small to filter with state-of-the-art filtration equipment) entering the chemical process stream; further, the rapid action of the pump (usually greater than 60 cycle per minute) creates massive impulse in the system with a resulting pulsed flow which forces particles through filters--thus rendering the filters ineffective. Finally, the mechanical shock inherent in this system creates constant maintenance problems.
The other system in general use today addresses only some of these problems. In the "pump/pressure delivery," a positive displacement pump is again employed to provide lift from the bulk source of chemicals. However, the chemicals are delivered to an intermediate vessel from which inert gas pressure is used to motivate chemical to the use areas.
Although the pump/pressure system is better controlled and is more conducive to use of filters to assure chemical purity, it still has serious drawbacks in a sub-micron chemical environment. Again, lift provided by a double diaphragm pump is restricted. Further, such pumps are prone to degradation--with the by-products entering the chemical stream. Finally, the use of a single pressure vessel for delivery means that delivery is not continuous, but is rather constrained to "batch" sizes based on the size of the pressure vessel. If demand exceeds the volume of the pressure vessel, further delivery must be "queued" while the pump refills the pressure vessel. Alternatively, pressure from the pump that is equal to or greater than the pressure of the delivery vessel must be applied to the delivery vessel to supplement or refill it during demand; this further compounds the filtration and maintenance problems.
Accordingly, it is a primary object of the present invention to provide a chemical transfer and delivery apparatus and method which effectively transfers high-purity process chemicals from any bulk source and delivers them accurately and without contamination to end-use stations.
It is an additional object of the present invention to provide such a transfer and delivery system which provides even flow at a consistent velocity so to permit accurate filtration and to minimize mechanical shock in the system.
It is a further object of the present invention to provide such a transfer and delivery system which does not employ pumps or other transfer apparatus which are subject to degradation or maintenance problems and which employs a minimum of any other moving parts which may be subject to degradation.
It is yet another object of the present invention to provide such a transfer and delivery system which has multiple flow paths so to provide virtually unlimited delivery capacity and built-in redundancy to avoid complete system shut down in instances of failure of a component of the system.